The present invention relates to an apparatus for assembling elevated structures and more particularly to a portable gantry crane apparatus which is useful in erecting and disassembling railroad bridges.
One commonly used type of crane system is known as a gantry crane. Referring to FIG. 1, a gantry crane 10 includes a plurality of vertical main support members 12, two main girders 14, a trolley girder 16 including parallel tracks 18 on its top surface and at least one trolley 20. The support members 12 support the main girders 14 which in turn support the trolley girder 16. Tracks 22 are also included on the upper surface of the main girders 14 so that the trolley girder 16 can move in either direction parallel to the main girders 14. The trolley 20 is fitted with wheels 24 so that it can move on tracks 18 parallel to the length of the trolley girder 16. A cable 26 extends down from a hoist on the trolley 20 and includes a block and tackle hook 28 at its lower distal end.
In operation, the trolley girder 16 can be moved on the tracks 22 so as to be above any area between the main girders 14. The trolley 20 can be moved so as to be anywhere along the length of the trolley girder 16. The hoist raises and lowers the cable 26 vertically displacing materials or equipment attached to the hook 28.
Gantry cranes are considered relatively safe for a number of reasons. For example, because the trolley girder 16 is supported by a plurality of support members and at least two main girders 14, load forces are distributed among a number of different girders and support members as opposed to a single boom. In addition, assuming that the maximum system load is not surpassed, there is little chance that gantry crane components will be driven into an unstable configuration where the load and system will be unbalanced. Furthermore, because the area between the main girders is free of obstruction, there is little chance that the trolley girder 16, the primary moving component of a gantry crane, will collide with other equipment.
While gantry cranes have many advantages, they are generally not suitable for on-site construction jobs. U.S. Pat. No. 4,497,153 describes one on-site gantry system which illustrates various problems that make onsite gantry systems impractical. The system includes two hoists positioned on, and movable along separate main girders. The hoists cooperate to move prefabricated beams laterally within the area defined by the main girders. The system, as is typical with all gantry systems, requires a complex configuration of support members and girders. Therefore, it is relatively expensive, difficult to transport, and requires a detailed and time consuming setup and takedown protocol.
In addition, the system is immobile after assembly. Thus, once assembled, the system can only transport equipment and materials within the area defined by the main girders. In order to use the system in another area, it must be disassembled, moved, and reassembled in the other area. These problems have generally limited gantry crane use to permanent operating areas or to small on-site areas.
Another common type of crane system is the mobile level-luffing crane. Referring to FIG. 2, a mobile crane 30 generally includes a carriage 32, a rotating machinery deck 33, operational machinery 34 supported on the deck 33, a hinged boom 36 attached to the machinery deck 33, a first set of topping lines 38, a second set of topping lines 42, and a hook block and tackle 40. The boom 36 is pivotally secured to the machinery deck 33 and operated by increasing and decreasing the length of the first set of topping lines 38. The second set of topping lines 42 is used to raise and lower the hook 40. The carriage 32 and deck 33 are ballasted, thereby adding stability to the crane when loaded.
In FIG. 2, the mobile crane 30 is a locomotive type, being self propelled and fitted with two railroad trucks 44. Power machinery to operate the mobile crane 30 is deck mounted, and the machinery deck is normally completely housed.
The mobile crane 30 overcomes many of the problems associated with a gantry system. For example, many mobile cranes are self propelled and can easily be moved to, and used at, on-site construction locations. In addition, it is not necessary to dismantle a mobile crane in order to move it around a construction site. In fact, often it is not necessary to dismantle a mobile crane to move it from one construction site to another. Furthermore, single beam boom construction makes the mobile crane a more economical option than a gantry system that requires a plurality of beams and support girders. These advantages make the mobile crane a particularly attractive option where crane functions are required for short periods at various construction sites or at different areas within a single large construction site.
Unfortunately, mobile cranes are relatively unsafe. For example, when a mobile crane is loaded and rotated laterally, often the ballast provided by the carriage and machinery deck is insufficient to maintain the crane in a stable position. When unstable, mobile cranes often tip causing damage to both the crane and surrounding structures, and often causing bodily injury to an operator.
Another problem with mobile cranes is that boom movement is not restricted. An unrestricted boom can be pulled back into a vertical position where it collapses over the machinery deck. In addition, as there is no guarantee that the area of boom operation will be free of obstruction, often a mobile crane boom will be mistakenly driven into other construction equipment or environmental structures, causing damage to the boom and other equipment.
While the industry has come up with various solutions to the mobile crane problems identified above, many of the solutions are relatively ineffective in certain industries. In particular, many of the solutions have not been effective in the railroad industry. For example, to stabilize a loaded mobile crane, outriggers or feet (not shown in FIG. 2) are provided which extend laterally from the carriage and contact the surrounding ground. In the railroad industry, while stabilizing outriggers can be used, the degree to which they extend laterally is limited by the construction of a railroad track. As most tracks are positioned on top of a berm, lateral extension is severely limited. Furthermore, as many berms are constructed of coarse rock, often the edge of a berm will be insufficiently stable to support a loaded outrigger. Thus, even when outriggers are used in railroad, because their lateral extension is limited, a mobile railroad crane will often tip when loaded and rotated laterally.
To eliminate the possibility of the boom collapsing over the machinery deck, the industry has come up with boom stops that limit the vertical positioning of the boom. In railroad however, a boom stop can tend to destablize a crane. The boom stop limits the boom to movement wherein the load is located a substantial distance from the ballasting machinery and deck. Because of the distance, the ballast has less stabilizing effect. This, combined with laterally restricted outriggers, results in a tippable and relatively unstable configuration.
The railroad industry uses cranes for many purposes. In particular, the railroad industry uses cranes to assemble, disassemble, and repair bridges on a regular and scheduled basis. Cranes must be used where building materials, such as prefabricated concrete girders, are extremely heavy. Rapid bridge replacement and maintenance is a high priority for any railroad, as train movement is effectively paralyzed when even a small span of track is inoperable. Thus, despite the mobile crane safety problems identified, railroads usually opt for mobile as opposed to gantry crane systems.
To minimize crane accidents, various procedures are regularly followed. For example, to minimize the lateral angle through which a mobile crane boom must rotate, bridge girders are normally pre-delivered to a construction site and placed at a pickup point adjacent a track in front of a train carriage. Because most berms are steep, the pickup point is usually located a substantial distance from the track on relatively flat and solid footing adjacent the berm. Often, where the footing is not solid, support piles must be driven into the footing to support the girders. To pick up materials, the boom must rotate at least partially laterally into a pickup position where the hook is over the pickup point.
Despite predelivery and efforts to limit lateral rotation, often a boom must be rotated substantially laterally in order to pick up a load. Careless operation under these circumstances has resulted in many tipping accidents.
U.S. Pat. No. 2,562,189 describes a gantry crane system which overcomes many of the problems associated with the swing crane systems and which is transportable. This system was designed specifically for transporting coffins and therefore has a relatively modest length. In addition, because coffins are relatively narrow and must often be transported through spaces not much wider than the coffin itself, the width of this system is particularly narrow. Due to its relatively modest dimensions, this system is light weight facilitating easy movement over the short distances typical in a cemetery.
Unfortunately, while this system is transportable, this system would be impractical for lifting and transporting large items such as bridge girders, prefabricated train tracks, or the like. In particular, if this system where adapted for travel along a railroad track, system width would be limited to the width of a typical track plus a typical lateral overhang on either side thereof. Railroad safety standards limit the maximum width of a railroad car to 9 feet, 2 inches (on an 89 foot car). Therefore, assuming a modest clearance of 1 foot, 3 inches for each girder, this system could not be used to pick up and transport bridge and track sections which have a width greater than 6 feet, 8 inches. Many track and girder sections have widths which exceed 6 feet, 8 inches. In fact, many bridge components have a width as wide as 14 feet. Therefore this system would be virtually useless.
Thus, it would be advantageous to have a bridge erection system that is mobile yet stable for transporting heavy construction materials to and from, and moving such materials at, construction sites wherein the width of the materials is equal to or slightly less than the maximum allowable safe transport width. In particular, it would be advantageous to have such a system for use in the railroad industry that could eliminate predelivery requirements, is relatively fast, safe, and efficient.